This invention relates to finely powdered high-purity fibroin and a process for producing the same.
Powdered silk fibroin is considered to be useful as an additive for cosmetic and pharmaceutical preparations, because of its moderate moisture absorption and retention properties and its high affinity for the human skin. Currently available silk fibroin powders are generally produced by finely dividing silk thread with pulverizer. Such a silk fibroin powder consists of filamentous fibers cut in very short lengths rather than nearly globular particles and, when used as an additive for cosmetic and pharmaceutical preparations, gives rise to various difficulties. For example, in mixing the powder with other ingredients in globular form, it is so liable to aggregation that a homogeneous final product is hardly obtained. Even if such a product is obtained, it shows poor slip properties upon application to the human skin and may occasionally produce round agglomerates of silk fibroin. Thus, it can be said that these difficulties prevent us from making good use of the excellent properties of silk fibroin.
With this background, the present inventors have made repeated studies on the production of a homogeneous fine powder of fibroin in globular particulate form rather than in fibrous form. In this field of art, for example, a process for producing silk fibroin suitable for use in chromatography is disclosed in Japanese Patent Publication No. 1941/'64. This process comprises dissolving silk fibroin in a cuprammonium solution or a solution of a copper complex (for example, a cupri-ethylenediamine solution), neutralizing the resulting solution with an acid, and adding an alcohol to the neutralized solution to form a white precipitate of silk fibroin. As a result of confirmatory tests made by the present inventors, it has been found that this process requires a very large amount of alcohol and, moreover, the resulting precipitate is too sticky to be separated by filtration. Another process for producing a powder of silk fibroin is disclosed in Japanese Patent Publication No. 4947/'51. This process comprises dissolving degummed silk fiber in a concentrated aqueous solution of a neutral salt such as calcium nitrate, dialyzing the resulting solution, and spray-drying the colloidal solution so formed. However, the powder of silk fibroin thus obtained is abnormally hydrophilic and, therefore, unsuitable for use as an additive for cosmetic preparations.
In order to overcome the above-described difficulties, the present inventors have made great efforts to produce an improved fine powder of fibroin in nonfibrous and particulate form, and have thereby completed this invention.